Conventionally, high-carbon ferrochromium is manufactured by smelting and reducing chromium ore after pretreatment in a submerged electric arc furnace (EAF) or the like. Examples of the pretreatment of the chromium ore include briquetting, sintering, pellet firing, and pellet pre-reduction.
In pellet pre-reduction, for example, the chromium ore is pulverized with coke and is granulated to prepare green pellets, which are then subjected to reduction roasting in a rotary kiln or the like at about 1,300 degrees C. or higher to provide pre-reduced pellets. The reduction degree of these pre-reduced pellets, which is 60% to 70% with only internally added coke, reaches 80% in combination with externally added coke. This method, therefore, has a significantly smaller amount of heat required for the reduction of chromium ore in the EAF than other types of pretreatment, thus greatly reducing power consumption.
Pellet pre-reduction is an advantageous method with low power consumption; however, this method, involving the use of a rotary kiln for the pretreatment, has the following problems unique to the rotary kiln. Because the fundamental principle of the rotary kiln is based on the tumbling of feedstock, the rotary kiln disadvantageously produces a large amount of dust that readily causes dam rings therein. In addition, the rotary kiln requires an excessive length due to variations in the residence time of the feedstock, thus involving a large equipment installation area and a large surface area. Consequently, the rotary kiln disadvantageously dissipates a large amount of heat, leading to higher fuel consumption than is desirable. Furthermore, a combination with externally added coke is disadvantageous in that it causes a large oxidation loss of the externally added coke in the rotary kiln.
Chromium oxide is reduced less easily than iron oxide from a thermodynamic perspective. The temperature of the pellets in the kiln is gradually raised by heating the pellets with a burner provided on a discharge end of the kiln. Accordingly, the internally added coke is consumed preferentially in the reduction of iron oxide contained in the chromium ore since iron oxide is reduced more easily than chromium oxide. As a result, the reduction of chromium oxide lags behind since chromium oxide is reduced less easily than iron oxide.
To solve these problems unique to rotary kilns, methods have been proposed in which a rotary hearth furnace (RHF) is used for the pre-reduction.
In one such method, green pellets, prepared by adding a carbonaceous material to a steel mill waste containing Cr and Fe and granulating the mixture, are preheated to about 600 degrees C. to 800 degrees C. with a shaft pre-heater, and are then charged into a rotary hearth furnace and gradually heated to about 1,000 degrees C. to 1,800 degrees C. in a reducing atmosphere.
In another such method, green pellets, prepared by adding a proper amount of chromium ore to a chromium-containing waste produced in the manufacturing process of stainless steel and granulating the mixture with coke, are placed on a hearth of a rotary hearth furnace and heated with a combustion gas to manufacture pellets containing chromium and iron.
The above methods, in contrast to rotary kilns, produce less dust and, therefore, cause no dam rings because the feedstock placed on the rotary hearth is stationary. In addition, no excessive hearth area is required since the residence time of the feedstock is uniform. Accordingly, the equipment used is more compact and the furnace surface area is smaller, such that the furnace has less dissipated heat and provides lower fuel consumption.
In the above methods, however, the internally added carbonaceous material starts to reduce iron oxide even at about 600 degrees C. to 800 degrees C. in the shaft pre-heater (while the carbonaceous material does not reduce chromium oxide at such temperatures). In addition, the pellets are gradually heated in the rotary hearth furnace; as a result, the carbonaceous material is consumed preferentially in the reduction of iron oxide. By the time the furnace reaches the temperature at which the reduction of chromium oxide can start, the chromium oxide loses the opportunity to come into contact with the carbonaceous material for lack of the carbonaceous material to give a low chromium reduction degree. On the other hand, increasing the amount of carbonaceous material added internally to maintain the contact opportunity causes the following typical problems: the green pellets disintegrate due to a decrease in strength to form deposits on the hearth; the dust loss from the rotary hearth furnace to the flue gas is increased; and the reduced pellets disintegrate, or otherwise their density decreases, to cause difficulty in dissolving in molten metal in the electric furnace, leading to a lower smelting yield.
Furthermore, the above methods make no mention of the heating temperature and temperature raising rate of the pellets and the above problem that the reduction of chromium oxide lags behind.
Accordingly, an object of U.S. Pat. No. 8,262,766 (Sugitatsu et al.), which forms a conceptual basis for some of the improvements of the present invention, for example, is to provide methods and systems for reducing a chromium containing raw material. When a chromium containing raw material that contains chromium oxide and iron oxide and is provided with an internally added carbonaceous material is reduced (i.e. pre-reduced), these methods and systems promote the reduction of chromium oxide, while suppressing the preferential consumption of the internally added carbonaceous material in the reduction of the iron oxide, thereby increasing the chromium reduction degree. However, these methods and systems also suffer from significant shortcomings, which are addressed by the methods and systems of the present invention, as described herein below.